Oil refineries, chemical plants etc are examples of places where electrical control equipment such as various sensors and displays are installed over wide areas including a lot of cabling. Grounding and insulation are critical concepts for such installations and thus a lot of rules as well as solid physical constraints exist. Further, the rules may differ between different countries.
“Grounding” is a very treacherous concept as it sounds far more easy than it is as applied on installation over long ranges or working at frequencies which are “high” as compared to the frequency corresponding to a wavelength equal to the range (100 kHz corresponds to one or a few km etc.). Three areas must be considered (together with functional requirements):                Physical facts tell that local grounding points spread over a large area (such as tens of meters to kilometers) will have different potential both for DC and AC (mainly 50/60 Hz) and that cables between such points will form possibly good (but unintended) antennas for various electromagnetic fields. In static conditions, levels above 1 volt are seldom experienced, but even so currents far above 1 mA can be measured if two separated “grounded points” should be connected by a typical cable. For transients far higher voltages can be experienced for instance at a lightening strike (many times being lethal for poor cows due to voltage transients between their four grounding posts).        Installation regulations (by law or by company rules) are not uniform in all countries and probably not optimal for all different situations. Protection of persons may be the most important goal, but undisturbed transmission of very weak signals may require other means. In some cases different solutions for a certain type of unit are required in different countries to follow the local rules and to get good function. For circuits, connected to mains, 1500 VAC rated voltage (1500 VAC test voltage between “hot” lines and ground) is a common requirement and in many cases minimum distances are specified between “hot” conductors as well as minimum thickness of insulation layers. Grounding of cable screens is different from grounding of circuits and can also be regulated.        Ex-regulations (Ex. abbreviation of: Regulations for Explosions Safety) have grounding requirements and also requirements for insulation to conductors, which are located in areas corresponding to “zone 0” which typically is inside a tank. Ex-regulations are many times not quite clearly formulated when it comes to practical cases but may in some respects appear as subject to negotiations. Insulation to ground for intrinsically safe circuits is in many cases specified as 500 V AC rated voltage, while insulation to parts in zone 0 (inside the tank) is more complicated. Typically one infallible or three good insulation components are to be used in the last case with some more or less obvious requirements for the good ones.        
The practical implementation for the grounding under different requirements is that the electronics shall be made to be well insulated from the local ground and then if required be connected in a suitable way different in different cases. For units using DC or very low frequencies (resistive temperature sensors, electro-mechanical level switches etc.) the galvanic concept “grounding” may be adequate, but due to capacitance in the cabling (in the order of 100 nF/km) all but very low frequency signals will have a rather complicated coupling to ground, the result of which is not always easy to predict.
For radar systems the situation is still more different as compared to units operating at low frequencies only. Many microwave (radar) circuits by themselves are locally grounded and to avoid disturbances, to enter (or escape from) these circuits, closed metallic enclosures are desired. “Locally grounded” means a fraction of a wavelength, which for radar frequencies is in the order of a single mm and “closed” for the radar frequencies means holes or slots smaller than a fraction of a wavelength. On the other hand the radar antenna has to be installed inside the tank. The combination of “good local connection to ground” and “good insulation from ground and from the interior of the tank” and “good transmission into the tank” makes a radar unit rather special with regard to the design of the insulation. Various solutions are in use and the fact that a physically small capacitance can have good microwave transmission properties is sometimes used.
One way to obtain insulation as seen from the field cabling is to use an AC power supply and an AC data transmission where transformers can be used for providing the gauge with both power and data communication but maintaining good DC insulation. The microwave circuit in that case can be locally grounded, or whatever, while maintaining good external insulation. In most cases for instruments in the process industry a system based on 4-20 mA DC data transmission is used and the corresponding insulation is more complicated or may degrade the accuracy. Sensors, which by their nature work, while they are enclosed in metal (temperature sensor, pressure sensor and other) may have an internal (and thus well protected) electrical insulation common for all components in the electronic unit.
In cases with high frequency signals entering the tank, the local signal grounding is less easy to use. An insulation on the high frequency side is then an alternative. One practical example of insulation to a wave guide according to prior art is shown in FIG. 1. A standard SMA-coaxial connector 1 is used as a feeding probe 2 in a wave guide 3 at for instance 10 GHz. A dielectric washer 4 under the connector forms a capacitor and by a conical recess 5 the distance between the metal parts is increased to withstand applicable test voltage (such as 500 VAC or 1500 VAC). The washer 4 must be thin to limit VSWR (Voltage Standing Wave Ratio) and due to the limited capacitance this solution is best for medium or high microwave frequencies (>5 GHz). A washer too thick as compared to the wavelength may create microwave matching problem and this especially at a frequency like 25 GHz (wavelength 8 mm).
On a printed circuit board (or corresponding ceramic carrier) an insulated coaxial connector can be located and insulated by capacitors as components. Tripled capacitors can be used to fulfil intrinsically safety requirements for DC Insulation while maintaining microwave coupling for frequencies in the lower microwave range.
In any of the cases a robust insulation, fulfilling a specified thickness of the dielectric layer like 0.5 mm, is hard to fulfil. Capacitors rated for high voltages are, due to their big size, difficult to combine with good microwave function.